bionic (2) umask.2.gz

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NAME

       umask - set file mode creation mask

SYNOPSIS

       #include <sys/types.h>
       #include <sys/stat.h>

       mode_t umask(mode_t mask);

DESCRIPTION

       umask()  sets  the  calling process's file mode creation mask (umask) to mask & 0777 (i.e., only the file
       permission bits of mask are used), and returns the previous value of the mask.

       The umask is used by open(2),  mkdir(2),  and  other  system  calls  that  create  files  to  modify  the
       permissions  placed  on  newly  created files or directories.  Specifically, permissions in the umask are
       turned off from the mode argument to open(2) and mkdir(2).

       Alternatively, if the parent directory has a default ACL (see acl(5)), the umask is ignored, the  default
       ACL  is  inherited, the permission bits are set based on the inherited ACL, and permission bits absent in
       the mode argument are turned off.  For example, the following default ACL is equivalent  to  a  umask  of
       022:

           u::rwx,g::r-x,o::r-x

       Combining  the  effect  of  this default ACL with a mode argument of 0666 (rw-rw-rw-), the resulting file
       permissions would be 0644 (rw-r--r--).

       The constants that should be used to specify mask are described in inode(7).

       The typical default value for the process umask is S_IWGRP | S_IWOTH (octal  022).   In  the  usual  case
       where the mode argument to open(2) is specified as:

           S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH

       (octal 0666) when creating a new file, the permissions on the resulting file will be:

           S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH

       (because 0666 & ~022 = 0644; i.e., rw-r--r--).

RETURN VALUE

       This system call always succeeds and the previous value of the mask is returned.

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

NOTES

       A  child  process  created  via  fork(2)  inherits  its  parent's  umask.  The umask is left unchanged by
       execve(2).

       It is impossible to use umask() to fetch a process's umask without at  the  same  time  changing  it.   A
       second  call  to  umask() would then be needed to restore the umask.  The nonatomicity of these two steps
       provides the potential for races in multithreaded programs.

       Since Linux 4.7, the umask of any process can be  viewed  via  the  Umask  field  of  /proc/[pid]/status.
       Inspecting  this  field  in  /proc/self/status allows a process to retrieve its umask without at the same
       time changing it.

       The umask setting also affects the permissions assigned to POSIX IPC  objects  (mq_open(3),  sem_open(3),
       shm_open(3)),  FIFOs  (mkfifo(3)),  and  UNIX domain sockets (unix(7)) created by the process.  The umask
       does not affect the permissions assigned to System V IPC objects created by the process (using msgget(2),
       semget(2), shmget(2)).

SEE ALSO

       chmod(2), mkdir(2), open(2), stat(2), acl(5)

COLOPHON

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